Corundum is aluminum oxide (Al₂O₃) with a hexagonal crystal structure, ranking 9 on the Mohs scale. Trace chromium produces ruby's red through absorption in the green-yellow region. Iron and titanium create sapphire's blue via intervalence charge transfer. Other colors arise from different impurities or combinations.
Corundum forms in metamorphic or igneous environments and shows strong pleochroism in many varieties. High hardness and toughness make it durable for jewelry. Heat treatment is common to improve clarity and color; unheated specimens are rare and highly valued.